The critical temperature of an MgB2 wire is 39K, which is higher by at least about 20K than that of usual metal type superconductive materials. Further, the upper critical magnetic field is also high and a thin film MgB2 has about 40 T. Further, since the magnetic anisotropy is extremely small and since it is excellent in bending property, it has little difference from usual metal type superconductive materials in the manufacture and handling of the wire materials. While various studies are being made at present for making MgB2 wire materials, it is considered that application of the superconducting wires under cooling by refrigerators with a smaller cooling capacity than that of a cryostat for cooling the conventional metal superconducting wires or in the intense magnetic field will become practical by utilizing the features. Among the application uses, products that require permanent current operation need superconductive connection.
Japanese Patent Laid-Open No. 2002-358972 (Patent Document 1) discloses a method of connecting a superconducting wire material formed by adding at least one of Y, Sn, Pb, Fe, and Al to MgB2, and a connecting portion wherein MgB2 powder is coated on the connecting portion and the wire and the connecting portion are bonded under pressure, and Japanese Patent Laid-Open No. 2003-037303 (Patent Document 2) discloses a method of connection the MgB2 superconducting wire and NbTi superconducting wire are bonded by bringing the superconductive cores into contact with each other upon removing a stabilizing material and coating a solder therebetween.
Generally, an MgB2 wire is manufactured by a powder-in tube method by filling MgB2 or Mg and B in a metal sheath tube such as made of Fe, SUS, Cu, Ni or a metal sheath tube formed of composite metals thereof, followed by fabricating and optionally a heat treatment.
In connecting the superconducting wires of MgB2, a method of directly connecting the MgB2 wire wherein the metal sheath tube at the connection portion is removed to expose the superconductive portion of MgB2 and the exposed portion is connected is disclosed in the Patent Document 2. However, the present inventors have found that this method and the method in Patent Document 1 are not suitable for the manufacture of the superconductive connection portion having excellent current conducting property and stability.
That is, when the metal sheath tube as a stabilizing material for the connection portion of the MgB2 wire is removed to expose MgB2, the denseness of the MgB2 wire, which is formed densely by the metal sheath tube, is lost and the superconductivity for the portion is remarkably deteriorated, thereby to form a connection portion that lacks the current conduction property and the stability.
The present invention aims at providing a connection structure of a magnesium diboride superconducting wire having excellent current conduction property and stability, wherein the superconductive connection between a magnesium diboride superconducting wire and a same or different kind of a superconducting wire are connected a method of connecting them.